Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss at beginning of period	$ (437)
Accumulated other comprehensive loss at end of period	(418)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	6,855		6,015
Total, pre-tax	(32,016)		(24,484)
Tax benefit	12,331		9,606
Amounts Reclassified from Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, net of tax	19
Gains and Losses on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss at beginning of period	(437)
Losses reclassified from net accumulated other comprehensive loss	19
Net current period other comprehensive income	19
Accumulated other comprehensive loss at end of period	(418)
Gains and Losses on Cash Flow Hedges [Member] | Amounts Reclassified from Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	32
Tax benefit	(13)
Total, net of tax	19
Gains and Losses on Cash Flow Hedges [Member] | Interest Rate Derivatives [Member] | Amounts Reclassified from Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	$ 32	[1]

[1] Included in interest charges.